Various methods are known as a method of color-image forming using the electrophotographic process. For example, in a first method, a plurality of monochromatic toner images of mutually different colors are formed on a toner image carrier. These toner images are consecutively transferred to one location on a recording medium in a superposed manner. In a second method, a plurality of monochromatic toner images of mutually different colors are formed on a toner image carrier. These toner images are then consecutively transferred to one portion of an intermediate transfer medium in a superposed manner. Subsequently, the superposed images are collectively transferred from the intermediate transfer medium to a recording medium. In a third method, toner images of a number of colors are formed directly on a toner image carrier. The toner images are then collectively transferred to a recording medium. These methods can be realized by an apparatus having only one light source for forming latent images. Further, these methods can also be realized by an apparatus having latent-image forming light sources in a number corresponding to the number of the toner colors. The latter type is referred to as a tandem type.
The above-described methods have both advantages and disadvantages. The first method excels in the transferability of the respective color toners. In addition, since the basic principle of the first method is simple, the first method has been widely put to practical use. However, development is substantially affected by the characteristics of the recording medium, and development on small-sized paper and thick paper is difficult. In the third method, the circumferential length of the toner image carrier is required to be as long as the length of the recording medium. For this reason, the apparatus becomes large in size. In addition, the formation of a latent image on the toner image carrier is influenced by the toner image which was earlier attached to and formed on the toner image carrier. For this reason, this method is inferior in the color reproducibility of a final image. Accordingly, in recent years, studies have been made on the second method using the intermediate transfer medium, and apparatuses based on this method have been commercialized. According to the second method, however, two transfers are effected before the toner images formed on the toner image carrier are transferred to the recording medium. For this reason, the operating characteristics of the transfer process largely affect the image quality.
To cope with this situation, there is a conventional example in which the arrangement of layers of the intermediate transfer medium is devised to prevent the occurrence of faulty transfer due to defects of the intermediate transfer medium itself as well as the resultant deterioration of image quality. Further, in another conventional example which is premised on the arrangement of the tandem type, an attempt is made to solve the problem in the following manner. Namely, the transfer rate of a toner image earlier transferred to the intermediate transfer medium and that of a toner image subsequently transferred thereto are set at the same rate. In this conventional example, the range of resistance values of the intermediate transfer medium and the range of voltages to be applied to the intermediate transfer medium are prescribed for this purpose.
However, with the method using the intermediate transfer medium, it is also necessary to take the following aspect into consideration. The characteristics of the toner image carrier change due to changes in the surrounding environment and extended periods of use. This also applies to the intermediate transfer medium and a transfer unit. For this reason, even if optimum materials and electrical conditions are set, if the settings are fixed, it does not follow that optimum image formation can always be effected. Excellent image formation is difficult particularly in a process which operates by a combination of members whose changes in characteristics vary. In order to effect excellent image formation, not only should transferability be excellent, but also the image quality of the toner images themselves which are formed on the toner image carrier should be set in optimum conditions. Further, the toner images themselves should be provided with excellent electrostatic characteristics which match the transferability. Nevertheless, in proposals and studies made concerning the conventional image forming systems, control values have been fixed with respect to various processes and members.